やや長周期の微動観測と地震工学への適用 (4)

Abstract

In the previous papers it was confirmed that the long-period microtremors change their spectral character very sensitively to deep soil deposits and also the amplification characteristic of earthquake strong motions is, specially in the long-period range, correlated to that of the microtremors. This clarified the engineering importance of observing the long-period microtremors. However, observational and analytical processes in those papers were concentrated to the elucidation of the spectral peaks of the microtremors and their relation to the underground structures, that is the microtremors were treated as if they were standing waves. For further improvement of their applicability in earthquake engineering, considerations from wave-theoretical point of view are better to be introduced. The best research way is to understand their generation, propagation, and amplification due to ground layers as a series of events.In this paper, as the first step of reserch in this way, a chiefly concerned investigation with the propagative characteristics of the long-period microtremors was carried out. To do this, a temporay array observation was prepared putting six sets of three component seismometers in an equal space of 60m, and the sequential observations were performed along three independent lines with an angle of 45° each other.Analysis was begun by obtaining apparent propagation velocities of the microtremors by means of phase correlation method, and by determining the direction of the wave propagation. Then the true phase velocities resulted as a function of frequency. Group velocities of the microtremors were roughly estimated by considering flows of the wave energies figured out on running power spectra at the two observation points far away from each other. Next, these phase and group velocities were compared with those calculated by knowing the underground structure at the site. Amplitudes of the microtremors in horizontal and vertical components were investigated in relation to their particle orbits at the ground surface.The results by these analyses are summarized as follows.1. The long-period microtremors are interpreted as an ensemble of dispersive waves. In the period range as we concern, the M11, and M21 modes of Rayleigh waves are dominant. Adding to these, the fundamental mode of Love waves is also mixed.2. The propagation direction of the long-period microtremors is, though slightly time-variant, decisive and mainly from open sea side (Pacific ocean).Finally, we would like to add that further experimental observations for the second step of the wave-theoretical analysis of the long-period microtremors are on schedule.